Liquid feeder for electrodischarge machining

ABSTRACT

A dielectric fluid system ( 2 ) for an electric discharge machining apparatus includes an auxiliary tank ( 22 ) positioned at or higher than a work tank ( 12 ) and a dielectric fluid reservoir ( 21 ), a filter device ( 23 ), and a first pump (P 1 ) and a second pump (P 2 ) upstream and downstream of the filter device  23 . Dielectric fluid in the dielectric fluid reservoir ( 21 ) is filtered by the filter device ( 23 ), and pumped to the auxiliary tank ( 22 ) using the first and second pumps (P 1 ) and (P 2 ). The filtered dielectric fluid is then rapid fed from the auxiliary tank ( 22 ) to the work tank ( 12 ). During electric discharge machining, dielectric fluid is pumped from the work tank ( 12 ), filtered, then cooled and circulated directly back to the work tank ( 12 ). A flushing pipe line and a third pump (P 3 ) may be added into the circulating pipe line. Also, a relief pipe line (K) containing a relief valve (RV) may be provided in parallel with the filter device ( 23 ), and a regulating pipe line (H), connected in parallel with the first pump (P 1 ) and the filter device ( 23 ), may be provided as an effective safety measure.

TECHNICAL FIELD

The present invention relates to a dielectric fluid system of anelectric discharge machining apparatus for collecting dielectric fluidfrom a work tank of the electric discharge machining apparatus cleaningthe fluid and re-supplying the fluid to the work tank.

DESCRIPTION OF THE RELATED ART

Electric discharge machining is a process for machining a workpiece intoa desired shape by periodically applying an appropriate electricdischarge machining voltage between the workpiece and a tool electrode(hereafter referred to simply as an electrode) arranged opposite to eachother across a specified electric discharge machining gap (hereafterreferred to simply as the machining gap) to continuously generateelectric discharges, and moving the workpiece and the tool electroderelative to each other. In order to be better able to carry out thiselectric discharge machining process, electric discharge machining fluid(hereafter referred to simply as dielectric fluid) is supplied to themachining gap.

The dielectric fluid plays several important roles as a machining mediumfor electric discharge machining. A first role is to remove chips anddebris generated as a result of electric discharges from the machininggap, as well as tar-like products which may become entrained in somedielectric fluids as a result of the heat due to the electricdischarges. A second role is to cool the machining gap so as to createfavorable conditions for the electric discharge machining process, andto suppress the tendency for deformation of the workpiece due to thermalexpansion. A third role is to maintain isolation of the machining gap inorder to maintain favorable electric discharge machining conditions. Aswill be described below, when an aqueous dielectric fluid is used, thespecific resistance of water is usually adjusted.

Therefore, the dielectric fluid used for electric discharge machining inthe work tank is filtered, cooled and subjected to specific resistanceadjustment as required, and then re-supplied to the work tank afterbeing collected in a dielectric fluid reservoir. Where an electricdischarge machine uses a flushing device such as a flushing nozzle,dielectric fluid is similarly supplied to a machining gap after beingfiltered. A typical electric discharge machining apparatus is thereforeprovided with a dielectric fluid system for supplying and controllingdielectric fluid.

A typical dielectric fluid system includes a dielectric fluid reservoirhaving a dirty dielectric fluid tank and a clean dielectric fluid tank.Dirty dielectric fluid that has been discharged from the work tank istemporarily collected in the dirty dielectric fluid tank, and chips anddebris having a comparatively heavy specific gravity precipitate here.The dielectric fluid in the dirty fluid tank is pumped through a filterapparatus by a pump, and chips and debris in the fluid are removed fromthe fluid which is then stored in the clean dielectric fluid tank.Decontaminated dielectric fluid in the clean fluid tank is re-suppliedto the work tank by a pump, and supplied to the machining gap forflushing, as required.

This type of dielectric fluid system may also include a dielectric fluidcooling apparatus, for keeping the dielectric fluid in the work tank ata specified temperature. Also, a dielectric fluid system of an electricdischarge machining apparatus which uses water based dielectric fluidmay be provided with, for example, a specific resistance controlapparatus including a deionizer using mixed-bed resin, which regulatesthe specific resistance of the dielectric fluid so as to maintain it ata value within a specified range.

The above-described conventional dielectric fluid system has severalshort commings which the present invention is intended to overcome.

A first object of the present invention is to reduce the time forsupplying dielectric fluid to an empty work tank (hereafter referred toas rapid feed), and more specifically to shorten the rapid feed time.Conventionally, when submerging a workpiece in dielectric fluid toperform electric discharge machining, the amount of dielectric fluid tobe supplied to the work tank is comparatively large, and the waitingtime until the work tank is filled is too long. Accordingly, anapparatus has been proposed to supply dielectric fluid to the cleandielectric fluid tank having a large capacity matching the work tank,arranged at a position higher up than the work tank, thus supplying thedielectric fluid to the empty work tank in a reduced amount of time.This is disclosed, for example, in Japanese laid open Patent No. Hei.5-004117 and Japanese laid open Patent No. Hei. 5-042424.

A second object of the present invention is to reduce the installationspace occupied by an electric discharge machining apparatus.Conventionally, in those electric discharge machining apparatus in whichthe workpiece is submerged in a dielectric fluid for machining, in orderto replace the workpiece after machining it is necessary to temporarilystore the dielectric fluid which is in the work tank in a dirtydielectric fluid tank having a capacity matching the volume of the worktank. Also, in order to supply cleaned dielectric fluid to the emptywork tank in a reduced time, it is also necessary to store, in the cleandielectric fluid tank, a volume of clean dielectric fluid which matchesthe volume of the work tank. As a result, the capacity of the dielectricfluid reservoirs for both the dirty dielectric fluid tank and the cleandielectric fluid tank is from 2.5 to 3.0 times the capacity of the worktank. This gives rise to a problem in that the installation spacerequired by the dielectric fluid system is quite large compared to theoverall installation space of the electric discharge machiningapparatus. As a countermeasure in order to reduce the installation spaceof the storage tank, the height of the storage tank is increased or thedirty dielectric fluid tank and the clean dielectric fluid tank arearranged so as to have a two-stage overlapping structure. This isdisclosed, for example, in Japanese laid open Patent No. Hei. 4-171123.

A third object of the present invention is to efficiently performcollection and filtration of some of the dielectric fluid in the worktank, and return it to the work tank during electric discharge machining(hereafter referred to as circulation). Conventionally, when thedielectric fluid reservoir is provided with a dirty dielectric fluidtank and a clean dielectric fluid tank, some of the dielectric fluid inthe work tank is collected in the dirty dielectric fluid tank, isfiltered by being passed through a filter, and is temporarily stored inthe clean dielectric fluid tank. Then, dielectric fluid in the cleandielectric fluid tank is circulated by being re-supplied to the worktank. In this case, as preparation for feeding clean dielectric fluid tothe empty work tank at the time of commencing the subsequent electricdischarge machining operation, it is necessary to store a large volumeof dielectric fluid, significantly more than the amount of dielectricfluid required for circulation, in the clean dielectric fluid tankduring the preceding electric discharge machining operation. For thisreason, a pump having a large discharge capacity and a filtration devicefor filtering a large volume are required, thus increasing costs. Also,particularly when water based dielectric fluid is used, the dielectricfluid comes into contact with more air while being stored in the largefluid tank, and there is a problem that the specific resistance value isreduced due to carbon dioxide penetration, etc. As a countermeasure, ithas been considered to directly collect dielectric fluid overflowingwhile performing electric discharge machining using a pump, filteringusing a filter, and to return the fluid directly to the work tank. Thisis disclosed, for example, in publications such as Japanese laid openPatent No. Hei. 5-037422, Japanese laid open Patent No. 8-215940 andJapanese Utility Model No. 2557992.

As described above, there are various problems in existing dielectricfluid systems which require improvement and solutions to some of theseproblems have been considered. However, none of the solutions proposedto date are adequate for solving all of the above-mentioned problems. Itis desirable to provide a dielectric fluid system which minimizes theinstallation space occupied by a dielectric fluid tank, efficientlyfilters and circulates dielectric fluid to supply clean dielectric fluidquickly to the work tank, and wherein, even if there is trouble with thepump of the supply device or clogging of the filter no major damage tothe dielectric fluid supply system will result.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide animproved dielectric fluid system for an electric discharge machine thathas a relatively small installation space, and also reduces the time forfeeding dielectric fluid to an empty work tank and that efficientlyfilters dielectric fluid circulating during electric dischargemachining.

It is also an object of the present invention to provide a dielectricfluid system for an electric discharge apparatus which implements safetymeasures to avoid major failures.

These and other objects and advantages of the present invention areachieved by the dielectric fluid system described below.

According to the present invention, a dielectric fluid system for anelectric discharge machining apparatus, for supplying dielectric fluidto a work tank with a workpiece submerged in the dielectric fluid maycomprise a dielectric fluid reservoir for storing dielectric fluidcollected from the work tank, an auxiliary tank positioned higher thanthe work tank and positioned at or above the height of the dielectricfluid reservoir, a filter device arranged between the dielectric fluidreservoir and the auxiliary tank, a main supply pipe line connecting thedielectric fluid reservoir to the auxiliary tank, and, provided inseries with a first pump, a filter device and a second pump, acirculating pipe line connected to the main supply pipe line at anintake side of the first pump and branching from the main supply pipeline at an outlet side of the second pump and connecting to the worktank, a rapid feed pipe line having a first drain valve for connectingfrom the auxiliary tank to the work tank, a drain pipe line having asecond drain valve for connecting the work tank to the dielectric fluidreservoir, first and second control valves positioned at an intake sideof the first pump for selectively connecting the main supply pipe lineand the circulating pipe line, and third and fourth control valvespositioned at an outlet side of the second pump for selectivelyconnecting the main supply pipe line and the circulating pipe line.

With this structure, clean dielectric fluid may be temporarily stored inthe compact filter device and the auxiliary tank arranged at or abovethe height of the dirty dielectric fluid tank. This which means thatthere is no need for a large, clean dielectric fluid tank as in therelated art. During advance preparation etc., dielectric fluid may bepumped to the auxiliary tank which is arranged at a position at or abovethe top of the work tank, and when the dielectric fluid is fed to thework tank, the dielectric fluid falls quickly from the auxiliary tank tothe work tank and at the same time deficient dielectric fluid, to theextent there is any in the auxiliary tank, is pumped from the dielectricfluid reservoir to the auxiliary tank. This means that the time neededto feed dielectric fluid to the work tank is effectively only the timeneeded to pump the deficient dielectric fluid to the auxiliary tank,thus further reducing the feed time.

A dielectric fluid system of an electric discharge machining apparatusof the present invention may further be provided with a flushing pipeline branching from the circulating pipe line at an outlet side of thesecond pump and connecting to a flushing device of the electricdischarge machining apparatus, and provided with a third pump and afifth control valve group inside the flushing pipe line for selectivelyopening and closing the flushing pipe line.

With this structure, a flushing operation in which dielectric fluid iscollected from the work tank, filtered, and jetted into the machininggap may be efficiently carried out by directly circulating a minimumamount of dielectric fluid.

A dielectric fluid system of an electric discharge machining apparatusof the present invention may further be provided with a by-pass pipeline, branching from the main supply pipe line at an intake side of thefirst pump, for connecting the flushing pipe line which includes thethird pump, at an intake side of the third pump, branching from theflushing pipe line at an outlet side of the third pump and connecting tothe auxiliary tank, and a sixth control valve for selectively openingand closing the by-pass pipe line.

With this structure, when the dielectric fluid system pumps dielectricfluid to the auxiliary tank, it is possible to use the third pump of theflushing pipe line in addition to the main supply pipe line pump, whichmeans that the rapid feed of dielectric fluid to the work tank can bemade still shorter.

Either of the dielectric fluid systems for an electric dischargemachining apparatus according to the present invention may be furtherprovided with a relief pipe line, including a relief valve, connectingan upstream side and a downstream side of the filter device and arrangedin parallel with the filter device, and a regulating pipe lineconnecting a branch point at an intake side of the first pump to aconfluence point at an intake side of the second pump, the confluencepoint being downstream of the filter device, and arranged in parallelwith the first pump and the filter device.

In this way, because the relief pipe line of the dielectric fluid systemis in parallel with the filter device and the regulating pipe line is inparallel with the first pump and the filter device, the first and secondpumps do not require an extremely large discharge capacity. In addition,even if the filter device becomes clogged, the filter will not rupture,and even if there is a failure of one of the pumps, there will be nooverall pump failure.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to theparticular embodiments shown in the drawings.

FIG. 1 is a front elevation showing an overview of an electric dischargemachining apparatus according to the present invention.

FIG. 2 is a plan view showing an overview of a dielectric fluid systemof an electric discharge machining apparatus according to the presentinvention, seen from above.

FIG. 3 is a cross section showing an overview of a filter device whichmay be used in the dielectric fluid system of FIG. 2, viewed from theside.

FIG. 4 is a piping layout drawing of a dielectric fluid system of anelectric discharge machining apparatus according to the presentinvention.

FIG. 5 is a piping layout drawing showing the state when the dielectricfluid system is pumping dielectric fluid from a dielectric fluidreservoir to an auxiliary tank.

FIG. 6 is a piping layout drawing showing the state when the dielectricfluid system is feeding dielectric fluid from an auxiliary tank to thework tank, and pumping dielectric fluid from the dielectric fluidreservoir to the auxiliary tank.

FIG. 7 is a piping layout drawing showing the state when the dielectricfluid system circulates dielectric fluid during machining, and forflushing.

FIG. 8 is a piping layout drawing showing the state when the dielectricfluid system is discharging dielectric fluid from the work tank, andpumping dielectric fluid from the dielectric fluid reservoir to theauxiliary tank.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of an electric discharge machining apparatus ofhaving a machining fluid according to the present invention is shown inFIG. 1, FIG. 2 and FIG. 3. Which the electric discharge machiningapparatus depicted is a wire cut electric discharge machine using awater based dielectric fluid, the same principles apply when theelectric discharge machining apparatus is a die sinking electricdischarge machining apparatus provided with a similar fluid tank andflushing device.

The electric discharge machining apparatus comprises the electricdischarge machining apparatus itself 1, a dielectric fluid system 2, apower supply device and a numerical control device for electricdischarge machining, although the latter devices are not shown in thedrawing. A table 11 is provided on a bed 3, and a work tank 12 formed offour wall members is arranged on the table 11.

The work tank 12 shown in FIG. 1 is shown in a state where a front dooris removed. In use workpiece WP (shown in dashed lines) is fitted to awork stand, not shown, which is positioned in the work tank 12. Also, asmall overflow chamber 12 b for collecting dielectric fluid that spillsover from the work tank 12, is formed by a partition baffle 12 a. Byadjusting the baffle 12 a up and down, it is possible to adjust theupper limit of the level of the dielectric fluid in the work tank 12.

Disposed above the workpiece WP, there is an upper guide assembly 13,including an upper nozzle UN for jetting dielectric fluid towards themachining gap during flushing operations, and an upper wire guide, notshown, for guiding a traveling wire electrode WE fed out from above. Theupper assembly is attached to a work head 4 via an upper arm. Also,disposed below the workpiece WP there is a lower guide assembly 14,including a lower nozzle LN for jetting dielectric fluid to themachining gap during a flushing operations, and an lower wire guide, notshown, for guiding the traveling wire electrode WE fed out from above.The lower guide assembly 14 is attached to a lower arm, provided on acolumn 5 and passes through the wall of the work tank 12.

The column 5 is moveably provided on the bed 3 to the rear of the worktank 12, and comprises a lower unit 51 that can move in the X axisdirection, and an upper unit 52 that can move in the Y axis directionorthogonal to the X axis direction. The work head 4 is attached to thiscolumn 5.

The dielectric fluid system 2 is typically positioned adjacent theelectric discharge machining apparatus 1. This dielectric fluid system 2comprises a dielectric fluid reservoir 21R, an auxiliary tank 22 mountedon this dielectric fluid reservoir, a filter device 23 and a coolingdevice 24 (hereafter referred to as a cooler). Also, as shown in FIG. 2,a first pump P1, a second pump P2 and a third pump P3, if required, areprovided in this dielectric fluid system 2. The piping, as well as thevarious control valves and sensors etc. appropriately arranged along thepiping are arranged, (not depicted in FIGS. 1-3) are shown in FIG. 4 andwill be described below.

In the illustrated embodiment, the dielectric fluid reservoir 21comprises only a dirty dielectric fluid tank. The dielectric fluidreservoir 21 comprises a first tank 21L, being an auxiliary tankprovided in a space in the bed 3, a second tank 21R being a main tankprovided in a lower section of the main body of the dielectric fluidsystem, and a linking tube 21P. These two sections of the dielectricfluid reservoir 21L and 21R are functionally the same fluid tank, but bydividing the dielectric fluid reservoir into two tanks the installationspace of the fluid tank 21R may be reduced to the extent of theinstallation area of the bed 3 side. The fluid tank 21L is positioneddirectly underneath the work tank 12, which means that by simplyproviding a drain opening in the bottom of the work tank 12 it ispossible to have an arrangement such that the comparatively largediameter drain tube 15 does not project outside of the electricdischarge machining apparatus main body 1. Naturally it is also withinthe scope of the invention that the dielectric fluid reservoir not bedivided and comprise only the second tank 21R. In other words, thepresent invention is not limited to dividing the dielectric fluidreservoir 21.

The capacity of the dielectric fluid reservoir 21 is best determinedwith reference to the maximum capacity of the work tank 12. The maximumcapacity of the dielectric fluid reservoir 21 is larger than that of themaximum capacity of the work tank 12, at least to the extent of thedielectric fluid overflowing from the work tank 12 and the circulatingfluid. The capacity may typically be about 1.2 times the maximumcapacity of the work tank 12. However, where the electric dischargemachine is a wire cut electric discharge machine, as in the illustratedembodiment, the dielectric fluid reservoir 21 is preferably formedhaving a capacity of about 1.5 times that of the work tank 12. This isbecause water based dielectric fluid evaporates during repeated use ofthe device and the amount of dielectric fluid initially supplied istherefore reduced over time. Also, an extra volume is advantageousbecause an amount of dielectric fluid equivalent to that retained in thebottom of the dielectric fluid reservoir is not discharged.

The auxiliary tank 22 is preferably provided on a platform of a frame 25arranged directly above the dielectric fluid reservoir 21, so that thebottom surface of the auxiliary tank 22 is positioned at least as highas the upper surface of the work tank 12. It is acceptable for thecapacity of the auxiliary tank 22 to be about half the volume of thework tank 12, as will be described later. In the illustratedarrangement, the auxiliary tank 22 is mounted with the dielectric fluidreservoir 21R, together with the pumps P1, P2 and P3, the filter device23 and the cooler 24.

The filter device 23 is advantageously mounted on a cover of the secondtank 21R of the dielectric fluid reservoir 21, in a space enclosed bythe frame 25. The filter device 23 filters dirty dielectric fluid,turning it into clean dielectric fluid, and supplies it downstream.

In more detail, the filter device 23, as shown in FIG. 3, preferablycomprises a filter 23 a, and a filter tank 23 b for storing the filter23 a. The filer 23 a filters and cleans dirty dielectric fluid pumpedfrom the dielectric fluid reservoir 21 and the work tank 12, and one ortwo filters are provided according to necessity. In this embodiment, aninternal pressure type filter is adopted as the filter 23 a. Thisprevents large force caused by pressure of dielectric fluid, which mayreach a maximum of 2 kgf/cm² in the event of a filter blockage, fromacting directly on the tank walls of the filter tank 23 b. The filtertank 23 b is a container for isolating filtered dielectric fluid fromthe outside in a fluid tight manner. As dielectric fluid from thiscontainer is quickly sucked in by the second pump 2, it is acceptablefor the filter tank 23 b to be only a small container.

A core tube 23 c is implanted in the center of the filter tank 23 b, anddielectric fluid supplied from the first pump P1 is pumped through theinside of the core tube 23 c, passes through minute holes in the coretube 23 c, is pumped to the inside of the filter 23 a, and is filteredby the filter 23 a. The filter tank 23 b is preferably tilted and anoutlet pipe opening 23 d is fitted at a lowermost part of the filtertank 23 b so that it is easy for the filtered dielectric fluid to becollected and sucked out. Also, a rail 23 e is preferably attached tothe inside of the filter tank 23 b, with the filter 23 a being mountedon this rail, so as to make changing of the filter 23 a inside thefilter tank easier. Reference numeral 23 f denotes an air purge. Whendielectric fluid is initially supplied after changing the filter 23 a,the air purge 23 f is opened so that air inside the filter tank 23 b canescape. However, if the volume of the filter tank 23 b is small, it ispossible to do away with the air purge. A drain hole, not shown in thedrawings, may be provided underneath the filter tank 23 b. When thefilter is changed this drain hole is opened and dielectric fluid insidethe filter tank 23 b is taken out.

A cooler 24 for cooling the dielectric fluid to a specified temperature,is well known in the art, and is preferably mounted on a platform 26provided at the rear of the frame 25 as depicted in FIG. 2.

FIG. 4 illustrates the piping arrangement for the dielectric fluidsystem depicted in FIG. 1 to FIG. 3. The state shown in this drawing,with dielectric fluid only retained in the dielectric fluid reservoir21, is the state of affairs immediately after initial supply ofdielectric fluid to the dielectric fluid reservoir 21, or when theelectric discharge machining apparatus has no immediate machining planand is stopped. A liquid level sensor S2 provided in the auxiliary tank22 detects that an appropriate amount of dielectric fluid has beensupplied to the auxiliary tank 22. Also, a liquid level sensor SIattached to an upper guide assembly 13 detects the height of the liquidlevel of dielectric fluid when dielectric fluid is supplied duringmachining to set the height of the liquid level. Even if the uppernozzle UN moves vertically during machining, the height of the liquidlevel of the dielectric fluid may be set in accordance with thismovement. Of course, it is also possible to use the baffle 12 adescribed in FIG. 1 to set the fluid level height. The pumps and controlvalves in the drawings are controlled by a control device, but thiscontrol device has been omitted from the drawings.

First, an outline of the six main piping paths will be described. Forconvenience, these paths are given functional names for ease ofunderstanding. As some of the piping paths may perform duplicatefunctions they will be given duplicate names.

Pipe line A is the main supply pipe line, connecting from the dielectricfluid reservoir 21 to the auxiliary tank 22, and includes the first pumpP1, the filter device 23, the second pump P2, and the cooler 24. Usingthis pipe line A, dielectric fluid is pumped from the dielectric fluidreservoir 21 to the auxiliary tank 22.

Pipe line B is a circulating pipe line, having the work tank 12 as astart point. Pipe line B is connected to the main supply pipe line A ata confluence point J1 upstream of the first pump P1 of the main supplypipe line A. It then branches from a branch point J8 downstream of anoutlet of the cooler 24 to return to the work tank 12. Large parts ofthe piping of pipe line B are common with the main supply pipe line A,sharing the pump P1, the pump P2 the filter device 23, the cooler 24,etc. Using pipe line B, dielectric fluid may be pumped from the worktank 12, filtered, then cooled and circulated back to the work tank 12.

Pipe line D is a rapid feed pipe line formed from a comparatively largediameter pipe, connecting a drain opening of the auxiliary tank 22 tothe work tank 12, and is used when dielectric fluid is fed rapidly tothe work tank 12.

Pipe line E is a drain pipe line for draining dielectric fluid of thework tank to the dielectric fluid reservoir 21.

Pipe line C is a pipe line for use where the event of the electricdischarge machine is equipped with a flushing device, and in thisembodiment it is a flushing pipe line, including the third pump P3,branching from a branch point J6 between the pump P2 and the cooler 24of the main supply pipe line A and connected to the upper nozzle UN andthe lower nozzle LN, being flushing devices. Using this pipe line,dielectric fluid for high pressure flushing is supplied to the uppernozzle UN and the lower nozzle LN. The third pump P3 is a so-calledhigh-pressure pump, capable of discharging dielectric fluid at acomparatively high pressure.

Pipe line G is a by-pass pipe line branching from a branch point J3upstream of the suction of the first pump P1 of the main supply pipeline A, connecting to the flushing pipe line C at a confluence point J9at the inlet side of the third pump P3, branching at the branch pointJ10 at the outlet side of the third pump P3, and finally reaches theauxiliary tank 22. This by-pass pipe line G is not required as a basicstructural element of the present invention, but providing this pipeline will make it possible to obtain better results. If this pipe line Gis provided, when dielectric fluid is pumped to the auxiliary tank 22using the main supply pipe line A, dielectric fluid can also be pumpedusing pipe line G, from the branch point J3 of the main supply pipe lineA, using the third pump P3 of the flushing pipe line C. The third pumpP3 can be used to obtain a reasonable discharge amount of low pressuredischarge which makes it useful for the above described pumpingoperation.

A description of the pump P1 and the pump P2 in the pipe line shared bythe main supply pipe line A and the circulating pipe line B now follows.

The first pump P1 is a main pump for pumping dielectric fluid from thework tank 12 or the dielectric fluid reservoir 21 and passing it throughthe filter device 23 for filtering. Accordingly, this pump requiressufficient discharge capability to pump a desired amount of dielectricfluid from the dielectric fluid reservoir 21 or the work tank 12, andconvey it to the filter tank against the filter resistance of the filter23 a, but does not need to be able to force dielectric fluid up to theauxiliary tank 22. The pump P1 therefore can be made smaller than a pumpdesigned to draw up dielectric fluid to the auxiliary tank after it hasbeen filtered.

The second pump P2 is provided on an outlet side of the filter device23, and is arranged in series with the first pump P1 to assist the firstpump P1. This second pump requires the discharge capability to pumpdielectric fluid filtered by the filter device 23, pass it through thecooler 24 and convey it to the auxiliary tank 22 or the work tank 12.Since the pump P2 only assists the first pump P1, its dischargecapability may be slightly smaller than that of the first pump P1, i.e.,a relatively small pump is sufficient. Also, since the pump P2 sucksdielectric fluid from the filter device 23, the filtering efficiency ofthe filter 23 a is improved. This improvement in filtering efficiencymakes fluid circulation more efficient by causing dirty dielectric fluidfrom the work tank 12 to pass directly through the filter device 23before returning it directly to the work tank 12.

Control valves are provided in the above-described pipe lines, as isdescribed below.

Constant flow valves L1 and L2 are respectively provided at the outletside of the first and second pumps P1 and P2. These valves respectivelyregulate the flow amount of dielectric fluid discharged from the pumpsP1 and P2, and prevent unreasonable loads or negative pressure frombeing imposed on the two pumps connected in series. Specifically, theflow amount of the constant flow amount valve L1 is preferably somewhatlarger than that of the constant flow amount valve L2, so that thedischarge amount of the pump P1 does not become unreasonably large, andso that negative pressure does not occur at the inlet side of the pumpP2. It is possible to adopt a flow amount control valve instead of theconstant flow amount valve. In this case, essentially the same type offlow control is carried out using the flow control valve. Check valves,(reference numerals omitted) are respectively arranged at least ateither a suction inlet side or a discharge outlet side of the pumps P1and P2. These check valves prevent reverse flow of dielectric fluid whenthe pumps are not operating.

In the main supply pipe line A and the circulating pipe line B, a firstcontrol valve V1 and a second control valve V2 are provided upstream ofthe confluence point J1, and a third control valve V3 and a fourthcontrol valve V4 are provided downstream of the branch point J8. Thepipe lines are opened and closed by these control valves, and the mainsupply pipe line A and the circulating pipe line B are changed over.Specifically, when dielectric fluid is pumped to the auxiliary tank 22,the control valves V1 and V3 in the main supply pipe line A are openedwhile the control valves V2 and V4 in the circulating pipe line B areclosed, and the main supply pipe line A is used. Also, when dielectricfluid is circulated, the control valves V1 and V3 are closed while thecontrol valves V2 and V4 are open and the circulating pipe line B isused.

In the flushing pipe line C a control valve V51 is provided downstreamof the branch point J6, a control valve V52 is provided downstream ofthe branch point J10 (required in case the by-pass pipe line G isadded), and a control valve V53 is provided downstream of the branchpoint J11, required where a wire threader jet pipe line M, issued, whichis described below, is added, as a fifth control valve group, and whenthese control valves are opened the flushing pipe line C is used.Specifically, the control valve V2 of the circulating path B is openedso that dielectric fluid is sucked from the work tank 12 using the firstpump P1, filtered by the filter device 23 and sent further downstream bythe second pump P2. Then after filtration clean dielectric fluid isconveyed to the flushing pipe line C, pressurized by the third pump P3,and finally supplied to the nozzles UN and LN, and to the machining gap.

A first drain pipe line D1 is provided in the rapid feed pipe line D,which is opened when dielectric fluid in the auxiliary tank 22,positioned above the work tank 12, is supplied to the work tank in ashort period of time.

A control valve, namely a second drain valve D2, is provided in the pipeline E, which is opened when dielectric fluid in the work tank 12 israpidly discharged to the dielectric fluid reservoir 21.

A control valve V6, being a sixth control valve, is provided downstreamof the branch point J10 of the by-pass pipe line G. The pipe line G isused with the control valve V52, one of the fifth control valve group ofthe flushing pipe line C, closed and with the control valve V6 opened.

Next, the piping paths relating to the safety measures of FIG. 4 will bedescribed. The pipe line H is a regulating pipe line, branching from thebranch point J2 at the inlet side of the first pump P1 to connect to theconfluence point J5 between the filter device 23 and the second pump P2,and makes the effective pipe line passing through the first pump P1 andthe filter device 23 shorter. In other words, from a point between thepump P1 and the pump P2, which are arranged in series, the regulatingpipe line H functions to return surplus dielectric fluid discharged fromthe pump P1, i.e., which is in excess of the amount for the pump P2, tothe inlet side of the pump P1. The regulating pipe line H also suppliesor re-circulates deficient or surplus dielectric fluid when it becomesimpossible to take out a consistent discharge amount from the pump P1 tofeed the pump P2.

For example, should the first pump P1 fail and it becomes impossible tosend sufficient fluid to the second pump P2, or should the filter 23 abecome clogged up, the regulating pipe line H supplies sufficientdielectric fluid directly to the second pump P2. Conversely, should whenthe second pump P2 fail and there is an excess of supply from the pumpP1, the regulating pipe line B re-circulates excess dielectric fluid tothe inlet side of the first pump P1. This avoids the occurrence ofserious, irreversible damage, such as cavitation due to negativepressure of the second pump P2, seizure due to no load, or overheatingdue to excess load of the first pump P1. Obviously the occurrence ofthese drawbacks can be controlled by conventional means (omitted fromthe drawings), such as sensing using pressure switches in the pipelines, or using thermal relays etc. to detect current overload of thepump motor and stop the motor, so that the pump may be stopped quickly.However, if the regulating pipe line H is provided, issues relating totoo much or too little fluid at the inlet to pump P2, continuing upuntil the pumps is/are stopped due to the above described control, willnot cause serious damage.

In this way, since the regulating pipe line H regulates the balance offluid between of the two pumps, P1 and P2 arranged in series with thefilter device 23, it has the effect of preventing overwork resultingfrom feeding fluid to the main supply pipe line A or to the circulatingpipe line B, even if the filter device 23 positioned between the twopumps is a small vessel or a sealed vessel. This is because it is notrequired that the filter device 23 adjust or buffer any imbalance in thefluid feed amounts between these pumps.

The pipe line K is a relief pipe line branching from the branch point J4between the first pump P1 and the filter device 23 to connect to theconfluence point J5 between the filter device 23 and the second pump P2.Pipe line K contains a relief valve RV. Should the filter 23 a becomeclogged and the pressure of the dielectric fluid raise to a specifiedhigh pressure, in the illustrated embodiment about 2 kgf/cm², the reliefvalve RV opens to link in the pipe line K. If a link is establishedthrough pipe line K, dielectric fluid discharged from the first pump P1passes through the relief valve RV and is fed directly to the inlet sideof the second pump P2. At this time, dielectric fluid that is fed fromthe first pump P1 and not sucked into the second pump P2 passes throughthe regulating pipe line H and is returned to the inlet side of thefirst pump P1, as explained above.

In this way, the relief pipe line K, acting in cooperation with theregulating pipe line H, substantially isolates the filter device 23 fromthe main supply pipe line A should the filter 23 a become clogged, thuspreventing explosive damage to the filter device 23, cavitation of thepump P2, or overheating of the pump P1 and the pump P2. The pipe line Khas the important role of ensuring safety of the filter device 23 andpumps P1 and P2 when dielectric fluid is pumped to the auxiliary tank 22or when dielectric fluid is caused to circulate to the work tank 12. InFIG. 4, the pipe line K merges at J5, the confluence point of the mainpipe line A and the pipe line H, but it is possible to have it merge ata position that is either upstream or downstream, but close to theconfluence point J5, as long as there is a connection between the filterdevice 23 and the second pump P2.

In FIG. 4, a number of pipe lines other than those described above arealso provided. Pipe line Q is an overflow pipe line for collectingdielectric fluid that overflows the liquid level set at a requiredheight by the baffle 12 a of the work tank 12 and directing the overflowto the dielectric fluid reservoir 21. Also, pipe line R is an overflowpipe line for collecting dielectric fluid, that has reached a high fluidlevel in the auxiliary tank 22 and has overflowed, and directing theoverflow to the dielectric fluid reservoir 21. The pipe line T is adrainage pipe line for draining dielectric fluid that has collected inthe filter tank, when the filter 23 a is replaced.

With the wire cut electric discharge machine of this embodiment thatuses water based dielectric fluid, specific resistance management pipelines are also required, in addition to the above described pipe lines.Pipe line N is a specific resistance management pipe line branching offat the point J6 between the second pump P2 and the cooler 24 andconnecting to the confluence point J7 at the outlet side of the cooler24. Pipe line N is provided with a deionizer 27, having an ion exchangeresin, and a control valve V8. A resistivity sensor is provided,preferably in the main supply pipe line A. The resistivity sensoroperates a resistivity meter IM, which provides an indication of thespecific resistance value of dielectric fluid passing through the mainsupply pipe line A, When this value is output to a control device, (notshown), the control device compares the detected value with a previouslyset specific resistance value. When the detected value is lower than thespecified set value, the control device opens the control valve V8 topass dielectric fluid through the deionizer, to adjust the specificresistance of the dielectric fluid, i.e., to make it higher.

A wire threader jet pipe line M for supplying dielectric fluid to anautomatic wire threader 6 is also provided in this embodiment. This pipeline M branches off at a branch point J11 immediately upstream of acontrol valve 53 of the jet pipe line C, and is connected to anautomatic wire threader 6. This pipe line is provided with a controlvalve V7. When the automatic wire threader 6 inserts a wire electrode WEfrom an upper guide assembly 13 into a lower guide assembly 14, thecontrol valve 53 is closed and the control valve V7 is opened to supplydielectric fluid to the automatic wire threader 6, and a jet stream forguiding the tip of the wire electrode in the insertion direction isproduced. At this time, the pressure of the dielectric fluid isincreased by means of the pump P3, as was the case for flushingdescribed above.

Besides the components described above, manual valves not shown in FIG.4 etc., which are only used in emergencies or for purposes ofmaintenance or inspection, and their operation, are omitted. Also, theabove described control valves for automatically opening and closing thepipe lines may be electromagnetic valves, air pilot valves, etc.

Next, the dielectric fluid supply process for the dielectric fluidsystem of the electric discharge machining apparatus of the presentinvention will be described using FIG. 5 to FIG. 8. In these drawings,pipe lines shown by a bold solid line are pipe lines mainly used, pipelines shown by a bold dotted line are pipe lines for auxiliary use, pipelines shown by a thin solid line are pipe lines that are used asrequired or used for maintaining safety, and pipe lines shown by adotted line are pipe lines that are not used. The control device causingoperation of the control valves and the pumps, and the control wiringbetween the valves and pumps and the control device, have been omittedfrom the drawings.

Before high speed supply of dielectric fluid to the empty work tank 12,that is, before rapid feed, a rapid feed preparation operation to pumpdielectric fluid to the auxiliary tank 22 is carried out. FIG. 5 showsthe state where this pumping operation is almost complete. When thecontrol device outputs a rapid feed preparation command signal, then, asshown in FIG. 5, the control valves V1 and V3 of the main supply pipeline A are opened and the first pump P1 and the second pump P2 aredriven to pump dielectric fluid from the dielectric fluid reservoir 21,and the fluid is filtered by the filter device 23, cooled by the cooler24 and pumped to the auxiliary tank 22. At this time, control valves V51and V52 used to may be select the flushing pipe line C, control valvesV2 and V4 may be used to select the circulating pipe line B, and thedrain valve D1 of the rapid feed pipe line D is obviously closed. Also,as long as blockage does not occur in the filter 23 a, the relief valveRV is closed and the relief pipe line K is closed. Dielectric fluid fromthe pump P1 side in excess of an amount pumped by the pump P2 circulatesin the regulating pipe line H. As required, the control valve V8 isopened to pass dielectric fluid through the deionizer 27, i.e., tomaintain the specific resistance of the dielectric fluid at a specifiedset value.

This rapid feed preparation operation has no relation to operations inthe vicinity of the work tank 12, and so it is possible to perform thisoperations during setting up operations, etc. In this case, the operatoris virtually does not experience any wasted waiting time due to thisrapid feed preparation operation.

In FIG. 5, in the event that the by-pass pipe line G is also provided,in addition to the pumping using the main supply pipe line A describedabove, dielectric fluid may also be pumped using the by-pass pipe lineG. Specifically, the control valve V6 may be opened and the third pumpP3 turned on to pump dielectric fluid to the auxiliary tank 22 throughthe by-pass pipe line G. By so doing, the amount of time required forpumping can be reduced in accordance with the amount of dielectric fluidpumped by the third pump P3.

When the fluid level sensor S2 detects the fluid level of the dielectricfluid in the auxiliary tank 22, the pumps P1 and P2 are stopped and thesystem then stands-by in this state. At this time, dielectric fluidoverflowing from the auxiliary tank 22 is returned to the dielectricfluid reservoir 21 from via the overflow pipe line R.

Next, as shown in FIG. 6, upon output of a rapid feed command signal thedrain valve D1 is immediately opened and dielectric fluid drops in one“slug” from the rapid feed pipe line D utilizing the difference ofelevation between the auxiliary tank 22 and the work tank 12. At thistime, the first and second pumps P1 and P2 of the main supply pipe lineare normally continuously driven, and dielectric fluid pumpedcontinuously in the auxiliary tank 22 drops down directly to the worktank. With this embodiment, since the volume of the auxiliary tank 22 ispreferably about half the volume of the work tank 21, the amount offluid pumped in advance to the auxiliary tank 22 is generally less thanthe amount of dielectric fluid that is to be rapid fed to the work tank12, which means that pumping for this insufficient amount is carried outcontinuously. Pumping time of the dielectric fluid is then sufficientlylonger than the time for dielectric fluid to fall, which means that thetime to continuously pump dielectric fluid to the auxiliary tank 22becomes the rapid feed time. Specifically, the rapid feed time is notthe time taken to supply all of a desired amount of dielectric fluid tothe work tank 12, but is the time taken to pump an insufficient amountof dielectric fluid, obtained by subtracting the amount of dielectricfluid held in advance in the auxiliary tank 22 from this total amount ofdielectric fluid, to the auxiliary tank 22.

With this type of rapid feed operation also, in a manner similar to thatperformed the time of the rapid feed preparation operation, dielectricfluid is circulated in the regulating pipe line H according to anydifference in discharge capability between the pump P1 and the pump P2.Also, the relief pipe line K may be provided in a closed state inreadiness for any abnormality. Pumping to the auxiliary tank 22, whichutilizes the by-pass pipe line G and the third pump P3, is preferablysupplemented. The specific resistance management pipe line N may also beopened as required.

We will now specifically described how the rapid feed time may be madeshorter. As previously mentioned, the volume of the auxiliary tank 22 ispreferably about half the maximum volume of the work tank 12. Assumingthat it is exactly half, a desired rapid feed time, a volume ofdielectric fluid to be supplied to the work tank 12, and the dischargecapability of the two pumps P1 and P2 may be selected, for example, asfollows. To rapidly feed dielectric fluid equal to about 400 liters,which is the maximum volume of a typical work tank 12, in about 5minutes, the volume of the auxiliary tank 22 is selected to be abouthalf the volume of the work tank 12, namely about 200 liters, and thedischarge capability of the pumps P1 and P2 is selected as about 40liters per minute. In this example, the volume of the filter tank 23 bis preferably about 50 liters.

In this example, the time required for rapid feeding of the maximumvolume of about 400 liters of dielectric fluid is slightly more than 5minutes, which is the time to pump the deficient 200 liters ofdielectric fluid to the auxiliary tank 22. Also, in the event that thedielectric fluid to be rapidly fed is 300 liters, the amount ofdielectric fluid that is deficient is only 100 liters, and so the finalrapid feed time in this case will be slightly longer than two and a halfminutes. As an extreme case, if the amount of dielectric fluid to berapidly fed is 200 liters, there is no deficit of dielectric fluid, andso the rapid feed time is only the time taken for the dielectric fluidto fall down from the above described drain, which in this embodiment isabout 20 seconds. On the contrary, in the event that the auxiliary tank22 is not used and the full amount of 400 liters of dielectric fluid isto be rapidly fed to the work tank 12 from an empty state, the rapidfeed time may be as long as about 10 minutes.

In the above-described embodiment, the rapid feed time is reduced toless than half as compared to the related art. Moreover, in the casewhere the dielectric fluid that is required to be rapidly fed is equalto the volume of the auxiliary tank 22, there is virtually no rapid feedtime required. Moreover, pumping to the auxiliary tank 22 is carried outduring set up, as a rapid feed preparation operation, which means thatthe only waiting time the operator experiences regarding dielectricfluid supply to the work tank, is the 1reduced rapid feed time.

In the event that the by-pass pipe line G is provided, in addition tothe rapid feed operation described above, it is possible to performpumping using the pump P3 of the pipe line G. In this specificembodiment, in the case where the feed capability of the pump P3 is 40liters per minute, the pumping capability to the auxiliary tank 22 isdoubled, and so the rapid feed time for supply of the 400 liters ofdielectric fluid, being the maximum volume described above, is reducedto about two and a half minutes. Also, in the event that the dielectricfluid to be supplied to the work tank is 300 liters, the feed time isreduced drastically to about one minute 15 seconds. In this way, byaddition of the by-pass pipe line G and the pump P3 the rapid feed timeis further reduced to less than a quarter of the related art.

Ultimately, by detecting the liquid level in the work tank 12 using theliquid level sensor S1 of the work tank 12, or by using the controldevice to measure fluid feed amount based on time to convey the fluid soas to estimate a high fluid level, the control device detects thedielectric fluid in the work tank 12 attaining a desired fluid amount,stops the first pump P1, the second pump P2 and the third pump P3, andcloses the respective control valves, thus completing the dielectricfluid rapid feed process.

Next, when an electric discharge machining start command is output bythe control device, then, as shown in FIG. 7 the control valves V2 andV4 are opened to form the circulating pipe line B, while at the sametime the control valves V51, V52 and V53 are opened to form the flushingpipe line C. The first pump P1 and the second pump P2 then feed dirtydielectric fluid from the work tank 12 to the filter device 23 and thecooler 24, and the dielectric fluid is filtered to produce cleandielectric fluid, cooled to a specified temperature. Then the clean andcooled dielectric is directly circulated from the circulating path B tothe work tank 12. When the dielectric fluid surface in the work tankbecomes lower during machining, as is indicated by a detection signalfrom the liquid level sensor S1 attached close to the upper nozzle UN,the control valve V1 is opened and an appropriate amount of dielectricfluid is replenished from the dielectric fluid reservoir 21 using thecirculating pipe line B, i.e., until the liquid level sensor SIgenerates a detection signal that the liquid level as appropriate. Also,in the event that the flushing pipe line C is provided, dielectric fluidis pumped from the work tank 12, using the circulating pipe line B, thenfiltered, pressurized by the third pump P3, and jetted to the machininggap from the nozzles UN and LN of the flushing pipe line C. Whenautomatic wire threading is carried out, the control valve V7 may beopened to supply dielectric fluid to the automatic wire threader 6. Asconcerns flushing, in the case where the by-pass pipe line G is providedat the inlet side of the third pump P3, hardly any dirty dielectricfluid is sucked from the by-pass pipe line G side, at the confluencepoint J9 of the flushing pipe line C and the by-pass pipe line G. Thisis because dielectric fluid inside the flushing pipe line C upstreamfrom the confluence point J9 is supplied with discharge pressure fromthe pump P2.

Thus, the amount of dielectric fluid filtered and supplied forcirculation and flushing from the circulating pipe line B and theflushing pipe line C is suppressed to the minimum fluid amount, andfiltration and cooling of circulated and flushed dielectric fluid iscarried out effectively. Obviously, surplus dielectric fluid alsocirculates in the regulating pipe line H during the above-describedcirculation and flushing operations, similarly to during the rapid feedpreparation and rapid feed operations. Also, the relief pipe line K maybe held in a closed (stand-by) state in readiness for any abnormality.The specific resistance management pipe line N may also be opened asrequired.

Upon completion of electric discharge machining, in FIG. 7, the controlvalves V2, V4, V52, V51 and V53 are closed, and the first pump P1, thesecond pump P2 and the third pump P3 are all stopped.

Next, when electric discharge machining has been completed and dirtydielectric fluid is discharged from the work tank 12, as shown in FIG.8, a drainage command is output from the control device to open thedrain valve D2 and dielectric fluid in the work tank 12 flows out fromthe drainage pipe line E to the dielectric fluid reservoir 21. Then,depending on whether or not subsequent electric discharge machining isto be resumed during or after the discharge operation, a commandindicating whether or not to resume pumping to the auxiliary tank 22 isinput. When electric discharge machining is not to be resumedimmediately, dielectric fluid is not pumped to the auxiliary tank 22 andis only stored in the dielectric fluid reservoir 21, and the system isthen in the state shown in FIG. 4. On the other hand, when machining isto be resumed urgently, then, after the discharge command or in additionto the discharge command, a rapid feed preparation command is outputfrom the control device. In this case, continuing on from a process forcompletely discharging dielectric fluid from the work tank 12, orconcurrently with this process, the control valves V1 and V3 are opened,and the first and second pumps P1 and P2 are operated to filter and coolthe dielectric fluid stored in the dielectric fluid reservoir 21 andpump this filtered and cooled dielectric fluid to the auxiliary tank 22.This state is the same as the above-described state in FIG. 5. Also, inFIG. 8, the rapid feed preparation state is shown in addition to thedischarge state. Use of the pipe lines H, K, N and G as required is alsothe same. After that, the system stands-by with the dielectric remainingstored in a specified amount in the auxiliary tank 22, and supply to thework tank 12 starts upon the next rapid feed command , as describedabove.

In this way, a process for pumping cleaned dielectric fluid to theauxiliary tank 22 can be commenced during the discharge operation orimmediately after the discharge operation, and upon completion ofoperations such as planning and preparation for carrying out the nextmachining etc., the system is placed in a state where cleaned dielectricfluid has been pumped to the auxiliary tank 22.

An important point of the invention disclosed in this embodiment is thatby replacing the clean dielectric fluid tank of the related art with thecompact filter device and the auxiliary tank, and by arranging theauxiliary tank above the dielectric fluid reservoir having only thedirty dielectric fluid tank, the installation space occupied by thedielectric fluid system is significantly reduced. Also, by arranging twopumps upstream and downstream of the filter device, and providing therelief pipe line K and the regulating pipe line H, highly efficientrapid feed and filtration may be enabled, even when using a small filterdevice, and the danger of serious damage occurring if there is cloggingof the filter or one of the pumps fails is all but eliminated. Also,during rapid feed, by continuously pumping dielectric fluid to theauxiliary tank in addition to allowing the dielectric fluid pumped inadvance fall down from the auxiliary tank, the time for rapid feed tothe work tank may be significantly reduced, even if the auxiliary tankis not as large as the work tank. Also, since the circulation of thedielectric fluid is carried out using direct circulation from the worktank to the work tank via the filter device, circulation can beperformed very efficiently with the minimum space. Further, in the caseof auxiliary pumping of dielectric fluid to the auxiliary tank via theby-pass pipe line using the pump of the flushing pipe line, the time forthis pumping may be dramatically reduced, as a result of which the rapidfeed time is shortened.

The above-described embodiments illustrate the presently preferredembodiments, and various modifications are possible within the scope ofthe present invention without deviating from the spirit of theinvention. Specifically, adding pipe lines or members having separatefunctions, changing the combination of pipe lines or the settingpositions of members, or the sharing of a number of pipe lines, isincluded within the technical scope of the present invention. Althoughin this embodiment a wire cut electric discharge machining apparatususing a water based dielectric fluid has been described as anillustrative example, the structure is basically the same with a diesinking electric discharge machining apparatus and/or an electricdischarge machining apparatus which uses an oil based dielectric fluid.

What is claimed is:
 1. A dielectric fluid system for an electricdischarge machining apparatus for supplying electric discharge machiningfluid to a work tank, comprising: a dielectric fluid reservoir forstoring electric discharge machining fluid collected from the work tank;an auxiliary tank, the bottom of which is positioned at least as high asa fluid level in the work tank and above the dielectric fluid reservoir;a filter arranged between the dielectric fluid reservoir and theauxiliary tank; a main supply pipe line connecting the dielectric fluidreservoir to the auxiliary tank, and including in series a first pumphaving an intake side and an outlet side, the filter, and a second pumphaving an intake side and an outlet side; a circulating pipe lineconnecting the work tank to the main supply pipe line upstream of theintake side of the first pump and branching from the main supply pipeline at the outlet side of the second pump, and connecting to the worktank; a rapid feed pipe line connecting to the auxiliary tank and thework tank and having a first drain valve; a drain pipe line, connectingthe work tank to the dielectric fluid reservoir and having a seconddrain valve; first and second control valves, positioned at the intakeside of the first pump, for selectively opening and closing the mainsupply pipe line and the circulating pipe line; and third and fourthcontrol valves, positioned at the outlet side of the first pump, forselectively opening and closing the main supply pipe line and thecirculating pipe line.
 2. The dielectric fluid system for an electricdischarge machining apparatus according claim 1, further comprising: arelief pipe line, including a relief valve, connecting an upstream sideand a downstream side of the filter; and a regulating pipe line,branching upstream of the intake side of first pump, and merging betweenthe filter and the intake side of the second pump, said regulating pipeline being connected in parallel with the first pump and the filter. 3.A dielectric fluid system for an electric discharge machining apparatusfor supplying electric discharge machining fluid to a work tank,comprising: a dielectric fluid reservoir for storing electric dischargemachining fluid collected from the work tank; an auxiliary tankpositioned at least as high as a fluid level in the work tank and abovethe dielectric fluid reservoir; a filter arranged between the dielectricfluid reservoir and the auxiliary tank; a main supply pipe lineconnecting the dielectric fluid reservoir to the auxiliary tank, andincluding in series a first pump having an intake side and an outletside, the filter, and a second pump having an intake side and an outletside; a circulating pipe line connecting the work tank to the mainsupply pipe line at a position upstream of the intake side of the firstpump and branching from the main supply pipe line downstream of theoutlet side of the second pump, and connecting to the work tank; aflushing pipe line branching from the circulating pipe line at theoutlet side of the second pump and connecting to a flushing device ofthe electric discharge machining apparatus, said flushing pipe linehaving a third pump having an intake side and an outlet side; a rapidfeed pipe line connecting the auxiliary tank and the work tank andhaving a first drain valve; a drain pipe line, having a first drainvalve, and connecting the work tank to the dielectric fluid reservoir;first and second control valves, positioned at the intake side of thefirst pump, for selectively opening and closing the main supply pipeline and the circulating pipe line; third and fourth control valves,positioned at an outlet side of the first pump, for selectively openingand closing the main supply pipe line and the circulating pipe line; anda fifth control valve group in the flushing pipe line for selectivelyopening and closing the flushing pipe line.
 4. The dielectric fluidsystem for an electric discharge machining apparatus according to claim3, further comprising: a by-pass pipe line, branching from the mainsupply pipe line at the intake side of the first pump, and combiningwith the flushing pipe line including the third pump at the intake sideof the third pump, and branching from the flushing pipe line at theoutlet side of the third pump and connecting to the auxiliary tank, anda sixth control valve for selectively opening and closing the by-passpipe line.
 5. The dielectric fluid system for an electric dischargemachining apparatus according to claim 4, further comprising: a reliefpipe line, including a relief valve, connecting an upstream side and adownstream side of the filter; and a regulating pipe line, branchingupstream of the intake side of the first pump, and merging between thefilter and the intake side of the second pump, said regulating pipe linebeing connected in parallel with the first pump and the filter.
 6. Thedielectric fluid system for an electric discharge machining apparatusaccording to claim 3, further comprising: a relief pipe line, includinga relief valve, connecting an upstream side and a downstream side of thefilter; and a regulating pipe line, branching upstream of the intakeside of the first pump, and merging between the filter and the intakeside of the second pump, said regulating pipe line being connected inparallel with the first pump and the filter.